[alarm circuit using lm555]

Can you go to this site and give me the comment?
http://www.technologystudent.com/elec1/thyris2.htm

[alarm circuit using lm555]

Thank you very much!
But what thing decide the frequency of buzzer?
In fact, I'm trying to understand the function of phase control of thyristor. Can you help me a simple circuit that features this function?

[alarm circuit using lm555]

This is an alarm circuit using IC lm555. But I don't really know the function of lm555.
Is it used for set the duration the buzzer will act.
Or to control the buzzer's frequency.
Still, it's said that thyristor in this circuit is not different from a switch which means not use its function of phase control. But I don't think so.
Can you explain it to me?